Booking.com has a “lowest price guarantee”. Expedia has a “best price guarantee”. Rentalcars.com says: “Best Car Rental Prices – Guaranteed!” But they are barely worth the pixels they are written on.

I can guarantee that, because I’ve just been booking my summer holiday in the Med. The hotel I wanted flashed up on Booking.com at £544, proclaiming that it was a “value deal” with only two rooms left. Plastered all over the site are various lures. “Lock in a great price ... prices may go up ... secure your reservation ... best price for two guests £544 ... best price guaranteed.” There’s even a crummy new line which says: “You’re saving an extra 10% on this hotel because, Patrick Collinson, you’re a booking Genius.”

It’s tripe, of course. I went on to the hotel’s own website last week and the cost was £466 – 15% less than Booking.com’s best offer. And, when I last checked, there were still rooms available.

The trust I once had in TripAdvisor is also fast evaporating. It now appears desperate to list the hotels or B&Bs where it has a deal to sell you a room, rather than bring your attention to what other travellers rate as best.

The B&B I had my eye on, top-ranked according to fellow travellers, fell towards the bottom of the page when I put in my dates. “We can’t find prices for this accommodation,” it said, while keenly making me aware of other hotels, where, presumably, it can earn a commission. When I rang the B&B I had no problem for the dates I wanted.

And who at TripAdvisor thought up “Just for You”. Whenever I search I’m no longer shown the top-rated hotels, but the ones that TripAdvisor thinks are best for me. I don’t know what algorithms it uses, but the ones it shows me are nearly always at the pricey end.

Now for car hire. I actually quite like the prices on Rentalcars.com, and have used it in the past. Its cheapest for six days from Marseille airport for the dates I wanted was £104.23. “This car is likely to sell out soon,” the site shouted. I’d better book now, because “a customer from the UK has just booked a car at this location” began flashing up in the corner of the screen. But over at Holiday Autos the same make, from the same rental provider, was £96.56 – some 8% cheaper than Rentalcars.com, despite the “best price guaranteed” line.

And flights? “Secure incredible value with Expedia’s Best Price Guarantee”, it says. The flight I wanted came up at £94.60. On the airline’s own site it was £89.60. These “best price” promises are, indeed, incredible.